12.1
Figure 1 White-tailed deer are becoming
more common in urban areas. Fish and
wildlife officers monitor deer populations
to maintain healthy, sustainable levels.
Population Characteristics
White-tailed deer (Odocoileus virginianus) are becoming more and more common in
some cities in southern Ontario. Urban infringement on natural habitats, increased
traffic, and changes in agricultural land-use practices have all increased interactions
between the deer and humans (Figure 1). The increasing presence of white-tailed
deer within city limits is problematic for both the deer and humans. Crop damage
and a growing number of collisions between deer and drivers have encouraged government and wildlife officials to initiate strategies to maintain healthy deer populations (groups of the same species living in a particular geographic area at the same
time) and reduce conflicts between the deer and humans.
Some people see the deer as destructive because the deer may damage crops or
create a hazard on roadways. They believe that culling the population (selectively
removing some of its members) is necessary. This was accomplished, in part, by a selective harvest system that was implemented in the 1980s to maintain healthy, sustainable
deer populations. However, other people recognize that interactions with the deer are
part of our relationship with nature. They believe that we should value the integrity and
beauty of nature, rather than regard nature as something to manage.
Monitoring wildlife and protecting the rich biodiversity of the province while providing ecological resources is part of the mandate of the Ontario Ministry of Natural
Resources. Wildlife officers monitor populations to determine if a species is invasive
or if it is a native species expanding its range because of human activities. They need
to decide what population size is ideal for a particular habitat and know when a
population has reached this ideal size. They must also maintain healthy, sustainable
population levels, using the many wildlife management options that are available.
CAREER LINK
population ecology a sub-field of
ecology that focuses on the dynamics of
populations and how populations interact
with their environment
Population ecology is the study of populations, particularly their size, density, dis-
tribution, and changes over time. Population ecologists monitor, evaluate, and model
changes in populations. By doing this, they are able to gather data that can help
them predict growth trends in populations. The data can also help them determine
the health of the different species and the ecosystems in which the species live. This
information then allows scientists to manage the size of the populations. Research in
ecology is often linked to work in genetics, physiology, anatomy, behaviour, palaeontology, and evolution, as well as geology, geography, and environmental science.
Many ecological phenomena, such as climate change, occur over huge areas and long
time spans, so ecologists must devise ways to determine how environments influence
organisms and how organisms change the environments in which they live.
The Distribution of Populations
The distribution of a population is described by three key factors: its geographic
range, defined by various boundaries; its population size and density; and the way
that individuals are dispersed through the area.
Geographic Range
geographic range the overall spatial
boundaries within which a population lives
habitat the place where an organism
normally lives
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The geographic range is the total area that is occupied by a population. Geographic
ranges vary enormously. The lakeside daisy (Hymenoxys herbacea), for example, is a
rare species in the aster family that lives only in the Great Lakes region. It is found
in Michigan, Ohio, Illinois, and southern Ontario. In Ontario, 38 populations of
this species are found in the southern part of Manitoulin Island and on the Bruce
Peninsula. Conversely, the tamarack larch (Larix laricina) has a very extensive geographic range. This tree is native to northern North America and can be found in all
the provinces and territories of Canada.
A habitat is a specific environment in which an organism lives. It is characterized
by its biotic and abiotic features. The common blue violet (Viola sororia), for example,
grows in shaded fertile habitats, where moist soil is prevalent. Since the violet grows
Chapter 12 • Natural Population Dynamics
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low to the ground, it tends to creep into lawns and gardens. Despite its pretty blooms,
it is often considered to be a weed. In contrast, the lakeside daisy is mainly found in
alvar habitats (limestone bedrock with thin or no soil). These habitats are threatened
by quarry and cottage development, as well as increased recreational usage. This
flower has been designated as a threatened species in an effort to stop its population
from declining.
Population Size and Density
Biologists are interested in population size (Nt ): the number of individuals that make
up a population at a specified time. They are also interested in population density (D ):
the number of individuals per unit area or per unit volume of their habitat. Species
with a large body size generally have a lower population density than species with a
small body size (Figure 2). For example, moose (Alces alces), a large herbivore that
may weigh as much as 700 kg, may have a home range between 5 and 40 km2. In
comparison, a snowshoe hare (Lepus americanus), a small herbivore that may weigh
up to 2 kg, may have a home range of just 0.1 km2. Thus, a single moose can use the
same amount of habitat as more than 400 snowshoe hares. Large animals require
more space than smaller animals because of their body size and food requirements.
population size (Nt ) the number of
individuals of a specific species that
occupies a given area or volume at a
given time
population density (D) the number
of individuals of the same species that
occurs per unit area or volume
Density (animals/km2)
1012
108
aquatic invertebrates
terrestrial invertebrates
mammals
birds
vertebrate ectotherms
104
1.0
10!4
10!9
10!6
10!3
Body mass (kg)
1.0
103
Figure 2 As the body size of animals increases, the number of animals per unit area tends to decrease.
The measure of population density can be misleading because of the unused
or unusable space within a habitat. Therefore, ecologists differentiate between the
number of individuals within an entire habitat (the crude density) and the number of
individuals per unit area that is actually used (the ecological density).
Although population size and density are related measures, knowing the density
of a population provides more information about the population’s relationship to the
resources it uses. Population density is calculated by dividing the total number of
individuals in the population (N) by the space occupied by the population (S):
N
S
For example, researchers identified 860 mature sugar maple trees (Acer saccharum)
growing in a 4 ha (hectare) forested area of central Ontario. The density of the trees
can be calculated as follows:
D5
crude density population density
measured in terms of the number of
organisms of the same species within
the total area of the entire habitat
ecological density population density
measured in terms of the number of
individuals of the same species per unit
area or volume that is actually used by
the individuals
860 trees
5 215 trees/ha
4 ha
In the same study, the researchers estimated that there were 75 000 maple seedlings
in a 0.5 ha plot. The density of the seedlings is
75 000 seedlings
5 150 000 seedlings/ha
0.5 ha
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A hectare is an area of 10 000 m2, so this value represents an average density of
15 seedlings per square metre. Clearly, the population of mature trees is less dense
than the population of seedlings. However, as the seedlings grow and compete with
each other and other species for nutrients, their density will more closely mirror the
density of the mature trees.
In addition to measuring the population densities of the mature sugar maples and
seedlings, the researchers also gathered data on red oak and beech trees in the same
area. By comparing the densities of different species over many years, the researchers
were able to document changes in the makeup of the entire ecosystem.
Dispersion of Populations
dispersion the pattern of distribution
in which a population exists; may be
clumped, random, or uniform
Populations can vary in their population dispersion, which is the spatial distribution
of individuals within a geographic range. Ecologists define three theoretical patterns
of dispersion: clumped, random, and uniform (Figure 3). Clumped dispersion is a
pattern in which individuals in a population are more concentrated in certain parts
of the habitat. It is a common pattern and occurs in three situations:
• Suitable living conditions are often distributed in patches. Cattails demonstrate
clumped dispersion because they are only able to grow in wet conditions, on
the edge of a lake or pond, or in wet soil.
• Populations of some social animals are clumped because mates are easy to
locate within groups. The animals may cooperate in rearing offspring, finding
food, and defending themselves from predators.
• Populations can be clumped due to limited seed dispersal or asexual
reproduction. For example, some species reproduce by asexual clones that
remain attached to the parents. Aspen trees exhibit clumped dispersion when
they originate asexually from a single plant. Clumping may also occur in
species in which seeds, eggs, or larvae lack dispersal mechanisms, such as
the ability to move on their own, travel in wind, or float on water. Therefore,
offspring grow and settle near their parents, forming clumps.
(a) clumped
(b) random
(c) uniform
A clumped dispersion pattern
is one in which individuals are
grouped more closely to each
other than if they were
randomly dispersed.
A random dispersion
pattern is one in which
organisms are distributed
independently of
each other.
A uniform dispersion pattern is
one in which individuals are
more widely separated from
each other than they are if
they were randomly dispersed.
Figure 3 (a) A clumped pattern of dispersion is evident in fish that live in social groups.
(b) A random pattern of dispersion, seen here in an Australian rainforest, is rare in nature. This
pattern serves as a yardstick for evaluating other dispersion patterns. (c) A nearly uniform pattern
is demonstrated by creosote bushes (Larrea tridentata) near Death Valley, California.
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Random dispersion occurs when environmental conditions do not vary greatly
within a habitat and individuals are neither attracted to nor repelled by others of their
species. In random dispersion, individuals are distributed unpredictably, and ecologists use statistics to determine when the distribution of a population is random.
Some spiders, burrowing clams, and rainforest trees exhibit random dispersion.
In contrast, individuals can be equally spaced throughout a habitat. This is called
uniform dispersion, and it may occur when individuals compete for food, breeding,
or nesting grounds. Creosote bushes are uniformly distributed in the dry scrub
deserts of the southwestern United States. Mature bushes deplete the surrounding
soil of water and secrete toxic chemicals, making it impossible for seedlings to grow.
Territorial behaviour, the defence of an area and its resources, can also produce uniform dispersion in some species of animals, such as in colonies of nesting birds.
Whether the spatial dispersion of a population is clumped, random, or uniform
depends partly on the size of the organisms and the size of the study area being examined. Oak seedlings may be randomly dispersed on a spatial scale of a few square
metres. However, over an entire mixed hardwood forest that covers many hectares,
they are clumped under the parent trees. Therefore, dispersion of a population
depends partly on the researcher’s scale of observation. CAREER LINK
The dispersion of an animal population often varies through time in response to
natural environmental changes. Few habitats provide a constant supply of resources,
such as food, throughout the year. Many animals move from one habitat to another
on a seasonal cycle, based on the distribution of resources. Tropical birds and mammals are often widely dispersed in deciduous forests during the wet season, when
food is widely available. During the dry season, these species crowd into narrow
“gallery forests” along watercourses, where evergreen trees provide food and shelter.
Studying Populations
Biologists measure population size and density to understand more about population
dynamics and to monitor and manage populations of endangered species, economically important species, and agricultural pests. For species of large animals, a simple
count of the number of individuals can provide accurate information. For example,
ecologists survey the size and density of populations of large African animals (such
as lions, wildebeest, and elephants) by aerial counting (flying over in an airplane)
(Figure 4(a)). This can be difficult, however, if the animals are camouflaged or concealed by trees (Figure 4(b)).
(a)
(b)
Figure 4 (a) Ecologists study populations of large African animals, such as elephants, using aerial
counting. (b) However, counting animals from the air can be difficult if they spend their time under
or in trees.
Often, the characteristics of a population make an accurate count impossible. Many
populations are dynamic, and their number and geographic location can change over
time. For these populations, scientists use sample data and their own observations to
estimate population size and density. For other populations, an estimate of size can
be made with indirect indicators, such as tracks, nests, or fecal droppings. However,
when accurate estimates of population size are important, or when biologists are faced
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587
with the challenge of populations that are mobile within their habitat or not readily
observable, such as populations of aquatic organisms, ecological sampling methods
are used. CAREER LINK
Sampling Populations
quadrat a sampling frame that is used for
estimating population size; the frame can
be real or virtual
Figure 5 Quadrats are used to sample
populations.
Population size and density are important population characteristics, but they are
often difficult to measure accurately. Many sampling techniques have been designed
for measuring populations. Each technique has its own strengths and weaknesses,
depending on the type of population that is sampled. A common and effective sampling technique for estimating population size and density of small or immobile
organisms is quadrat sampling.
A quadrat is a square made of wood, plastic, or metal that is used to isolate a
sample (Figure 5). The size of the quadrat depends on the size of the population
being sampled. For example, if a researcher wanted to estimate the population size
of dandelions growing in a meadow, the quadrat would likely be 1 m2. However, if
the individuals in the population were much larger, the size of the quadrat would
increase.
Quadrats must be placed randomly in the area being sampled to avoid bias. They
must also be representative of the entire area in question. If a biologist who was determining the density and size of plants in a schoolyard placed quadrats only along a
path, for example, the samples would not be random and therefore not representative
of the entire population. Bias such as this can be avoided if the quadrats are placed at
randomly chosen coordinates. In the tutorial below, you will practise using quadrats
to estimate population density.
Tutorial 1 Estimating Population Density
Quadrat sampling is one of the most convenient techniques for measuring the abundance
of organisms in a specific area.
Sample Problem 1: Using Quadrats to Estimate Density
A biologist wants to estimate the population size and density of snails on a beach. The
beach measures 100 m by 50 m. The biologist randomly places four 1.0 m by 1.0 m
quadrats on the beach. Estimate the population size and density if the biologist counts
55, 13, 42, and 35 snails in the four quadrats.
Solution
You can estimate the population density of the snail by calculating the average density of
the sampled quadrats.
estimated population density 5
5
total number of individuals
sampling area
55 1 13 1 42 1 35
11.0 m22 1 11.0 m22 1 11.0 m22 1 11.0 m22
5 36.2 snails/m2
Assuming that the sample is representative of the total study area, the estimated population
density of the snails is 36.2 snails/m2.
You can estimate the population size by multiplying the estimated population density by
the total study area.
estimated population size = (estimated population density) (total size of study area)
= (36.2 snails/m2)(5000 m2)
= 181 000 snails
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Practice
1. In a study of eastern red-backed salamanders (Plethodon cinereus), a biologist
randomly placed seven 1.0 m by 1.0 m quadrats in a field that measured 100 m by
100 m. Estimate the population density and size if the biologist found 0, 1, 1, 2, 0, 1,
and 3 salamanders in the seven quadrats. [ans: 1.1/m2, 11 000]
2. In a survey of northern mountain ash seedlings in northern Ontario, researchers
randomly placed eight 1.0 m by 2.0 m quadrats in a field that measured 100 m by
150 m. Estimate the population density and size if the researchers found 2, 0, 0, 8, 6,
1, 1, and 3 seedlings in the eight quadrats. [ans: 1.3/m2, 20 000]
MARK-RECAPTURE SAMPLING
Some animal species continually move around, seeking protection, food, and shelter in
hard-to-see areas. These species are difficult, if not impossible, to count. One common
sampling technique that is used to estimate the population size of these species is the
mark-recapture method. A specific number of animals in the natural population is captured, marked, or tagged in some way and then released back to the population. After
a period of time, when the marked animals have mixed with the unmarked animals,
another sample is captured. Biologists use the proportion of marked to unmarked
animals in the second sample to estimate the size of the entire population.
The accuracy of this method is dependent on five assumptions:
mark-recapture method a sampling
technique for estimating population size
and density by comparing the proportion
of marked and unmarked animals that
are captured in a given area; sometimes
called the capture-recapture method
• The chances for each individual in the population to be caught are equal and
constant for both the initial capture and the recapture. Marked individuals
must not be or become easier or more difficult to catch.
• The proportion of marked to unmarked animals remains the same between
captures. No new individuals can enter the population through birth or
immigration, and no individuals can leave through death or emigration.
• Enough time is allowed between the initial capture and subsequent recapture
for all the marked individuals to disperse randomly throughout the population.
• The captured animals are not adversely affected by their marks.
• The marked animals do not lose their marks.
Based on these assumptions, the estimated population size can be calculated as follows:
number of recaptures 1m2
total number marked 1M2
5
total population 1N2
size of second sample 1n2
or
Mn
N5
m
where
N = estimate of the total population size
M = total number of animals captured, marked, and released on the first visit
n = total number of animals captured on the second visit
m = number of marked animals that were recaptured on the second visit
In the following tutorial, you will estimate population sizes based on data from
mark-recapture sampling.
Tutorial 2 Estimating Population Size
The mark-recapture sampling method is used to measure the population size of large or
highly mobile animals.
Sample Problem 1: Mark-Recapture Sampling
Biologists were studying a saw-whet owl population of unknown size. They captured,
banded, and released 30 individuals. They waited until they assumed that the released
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12.1 Population Characteristics
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individuals had moved randomly through the population. Then they captured a second
sample of 80 individuals and found that 12 individuals were marked. Use these values to
estimate the population size.
We can assume that the ratio of marked individuals to the size of the total population
equals the ratio of marked individuals recaptured in the second sample.
N5
Mn
m
N5
30 3 80
12
N 5 200
Therefore, the estimated population size is 200.
Practice
1. Biologists captured 75 red foxes in a forested area and marked them with ear tags.
Exactly 10 days later, they recaptured 45 red foxes and found that 15 had ear tags.
Estimate the red fox population in this area. [ans: 225 red foxes]
2. In a northern Ontario lake, 208 lake trout were captured and marked with a tag on
one fin. Two weeks later, 185 lake trout were recaptured and 13 had tags. Estimate
the lake trout population in this lake during the study period. [ans: 2960 lake trout]
3. Imagine that you capture a sample of 180 butterflies in a park, mark them, and
then release them. A week later, you recapture 210 butterflies and find that 30 are
marked. Determine the estimated butterfly population in the park during the study
period. [ans: 1260 butterflies]
Tracking Populations
Tracking animals provides useful information about their behaviour. Animals can be
tracked with special transmitters using Global Positioning System (GPS) technology
and satellites. They can also be tracked with DNA sampling, by collecting and testing
their droppings and other biological materials. Biologists have been able to learn a
great deal about the range, distribution, and population density of various species
with these techniques. For example, satellite tracking of 25 female sea turtles showed
that their migrations followed three major routes from Gabon in Africa (Figure 6).
This research was vital for determining which conservation strategies should be used
for the sea turtles. WEB LINK
Africa
Group I
South
America
Group II
South Atlantic
Ocean
Group III
Figure 6 Satellite tracking of sea turtle migrations off the western coast of Africa
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Another study, called the Y2Y project (for “Yukon to Yellowstone”), started with a
female wolf nicknamed “Pluie” (after the French word for “rain”). She was collared by
researchers in June of 1991 in Alberta’s Peter Lougheed Provincial Park and tracked for
two years. Satellite locations recorded Pluie’s travels. In a nine-month period, she wandered over 103 000 km2, an area 15 times as large as Banff National Park, and she spent
time with five different wolf packs. In her travels, she passed through three states, two
provinces, private lands, and First Nations, Inuit, and Métis lands (Figure 7).
Movements of Pluie
Pluie captured and collared
June 6, 1991
GPS collar locations
Banff
Calgary
ALBERTA
BRITISH
COLUMBIA
Fort
Macleod
Medicine
Hat
Fernie
Nelson
WASHINGTON
MONTANA
Spokane
IDAHO
0
50
100
150
200 km
Great
Falls
Missoula
Helena
Figure 7 Pluie travelled through Banff National Park into southwest British Columbia, then into
Montana, Idaho, and Washington state before returning to British Columbia.
The Ethics of Studying Populations
Marking and tracking can be excellent techniques for monitoring and sampling a
population. However, there is an ethical debate about their potential impact on the
animals involved. Some scientists have expressed concern that the handling of animals during data collection for population density and size may alter their behaviour
after their release or reduce their reproductive ability. Other scientists are concerned
that trapping methods may be harmful. The long-term effects of repeated tranquilizing of large animals are not known, nor are the effects of collars and tags. The effects
of pursuing, capturing, and marking animals are also not known. However, a recently
published 10-year study of banded King penguins by French researchers has shown
that banding the penguins on a flipper has reduced their lifespan and their reproductive fitness.
The Canadian Council on Animal Care (CCAC) is in the process of developing a
set of guidelines for ethical wildlife research. The CCAC is encouraging researchers
to reduce their use of animals in studies as much as possible, to support and develop
techniques that will minimize pain and distress in animals, and to replace trapping
with computer estimations wherever possible.
Measuring and estimating population size are necessary for developing population
management strategies and reducing the effects of environmental stressors, whether
natural or human-made. However, efforts should be made to ensure that the use of
animals in research is valid, humane, justifiable, and considerate.
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12.1 Review
Summary
• Biologists use measurements, such as population size and population density,
to describe and monitor populations. These measurements also help biologists
manage species.
• Populations in a given geographical range show one of three dispersion
patterns: clumped, uniform, or random.
• Biologists use quadrat sampling and mark-recapture sampling to estimate
population size and density.
• Satellite and other technology can be used to track animals that migrate or
move over long distances.
• Ethical issues and concerns need to be considered when studying and
monitoring wildlife populations.
Questions
1. (a) What are the three types of dispersion?
(b) Do dispersion patterns of populations always
stay the same? Why or why not? K/U
2. Use the Internet to find satellite tracking data for
a species of wildlife, and answer the following
questions. Be sure to include the URL of the site
T/I
A
you used to answer the questions.
(a) What animals are being tracked?
(b) How far did the animals travel?
(c) Where were they located?
(d) How are the data being used by scientists?
(e) How can such data help to protect endangered
species?
3. Using a Venn diagram or t-chart, compare and
contrast quadrat sampling and mark-recapture
sampling for estimating population size. What
underlying assumptions must be made for each
technique to be effective? K/U C
4. Research the invasive plant species called garlic
mustard (Alliaria petiolata) and the impact it is
T/I
having in Ontario.
(a) When and how was garlic mustard introduced
into North America?
(b) Identify the major problem(s) it causes for
native species.
(c) What, if anything, is being done to stop its
spread?
5. Researchers captured 18 black bears in a large
forested area. They marked and released the bears.
In a second trial, they captured 16 black bears and
found that 5 had been marked. Estimate the overall
population of black bears in this area. T/I
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Chapter 12 • Natural Population Dynamics
6. To estimate the size of a dandelion population in
a schoolyard, a biology student randomly placed
five 1.0 m by 1.0 m quadrats in a site that measured
40.0 m by 40.0 m. The numbers of dandelions in the
quadrats were 6, 10, 2, 4, and 7. K/U T/I
(a) Estimate the density and size of the dandelion
population in this study site.
(b) What assumptions did the student have to
make when estimating the density and size of
the population?
7. A biologist wanted to estimate the density and
size of a burrowing prairie dog population in a
grassland habitat. The biologist had to choose
between quadrat sampling and mark-recapture
sampling. T/I A
(a) Which sampling technique do you think would
be easier to implement, and why?
(b) Which sampling technique do you think would
be more effective, and why?
8. A team of wildlife researchers plans to study
caribou that live in the Arctic tundra using markrecapture sampling. T/I
(a) What challenges might the researchers
experience when estimating the density and
size of the caribou population? Is there a better
sampling technique that could be used for this
population?
(b) What are the possible potential short-term
and long-term effects of studying the caribou
population?
WEB LINK
NEL